Process description: Plating chromium
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- Chromium plating
Chromium plating has found wide usage both as a decorative surface finish (bright chromium plating) and as a functional coating (hard chromium plating), because of its typical high hardness and wear resistance properties. It is also widely used in packaging applications.
For decoration, often referred to as bright chrome or bright chromium, it is usually applied as a thin layer to prevent the corrosion of the very level and bright surfaces generated by bright nickel undercoats. Deposit thickness is generally in the range 0.1 – 0.4 μm, with a treatment time from 2 to 13 minutes. The finish has a typical silver-bright colour and has a very high resistance to tarnishing.
Bright chromium can be plated either from a hexavalent or trivalent chromium electrolytes.
Hard chromium plating (usually known as hard chrome) consist of heavy deposits applied on particular components (drive shafts, hydraulic cylinders, aircraft landing gear, pins, valves, etc.) to give high resistance to mechanical and wear damage. Hard chromium plating can only be plated from hexavalent chromium electrolytes.
Aerosols are generated from the hexavalent process solution by significant cathodic hydrogen evolution. Theoretically, trivalent processes based on a chloride solution may produce hazardous organic halogens (AOX) and chlorine gas, but production solutions prevent this by additives reducing the chlorine. There is no problem with sulphate-based solutions.
- Bright chromium plating (hexavalent chromium electrolytes)
Bright hexavalent chromium plating electrolytes are based on chromic acid (80 – 400 g/l), sulphate as the primary catalyst (0.8 – 5.0 g/l) such as fluoride ions (<2% of the concentration of the chromic acid). Where high corrosion protection is required so-called “micro-cracked or micro-porous” chromium coating can be applied using readily available technique, with a thickness from 0.7 to 0.8 μm, and a treatment time of 7 – 8 minutes.
Decorative chromium coating properties are determined by the characteristics of the nickel underlayer, by the CrO3/catalyst ratio and by the operating temperature (20 – 45°C).
- Bright chromium plating (trivalent chromium electrolytes)
Bright trivalent chromium electroplating electrolytes are based on chromium III compounds, such as sulphate or chloride, together with proprietary chemicals. The electrolyte contains only about 20 g/l of the trivalent chromium, compared with about 200 g/l of chromium acid in the hexavalent chromium process.
Currently, trivalent chromium can only be used for decorative finishes, and cannot replace hexavalent chromium for hard chrome plating.
The use of trivalent chromium eliminates the carcinogenic and other hazards associated with hexavalent chromium in the workplace. Fume extraction and scrubbing, or fume suppressant are not required for hexavalent chromium. However additives are required to prevent the formation of free chlorine and AOX.
The lower electrolyte concentration has a lower viscosity than the hexavalent electrolyte. This results in better draining of plated part, and subsequently less drag-out, less loss of electrolyte, less effluent treatment required and less chromium-containing waste being produced.
- Black chromium plating
Black chromium finishes can be achieved for decorative black pieces and on the same substrates as for bright chromium plating. They are also plated onto a preceding nickel layer. Usually, they are treated in emulsions to achieve a decorative finish after the plating process. They are based on hexavalent chromic acid electrolytes (350 – 520 g/l) and catalysts (nitrates, fluorides). The layers are porous and <1 μm.
- Hard chromium plating
Hard chromium plating electrolytes are based on chromic acid (180 – 350 g/l) and on one of the following catalysts:
- sulphate ions (1.8 – 6.0 g/l)
- mixed sulphate and fluoride ions (<2% of the content of the chromic acid)
- pre-prepared proprietary fluoride-free (<2% of the content of the chromic acid)
The catalyst choice is fundamental to the efficiency of the electrolyte (from 25 – 33% for the proprietary fluoride-free type). The type of catalyst used, and the operating temperature have a great influence both on the physical properties (cracked, micro-cracked and crack-free coatings) and on the chemical and mechanical properties, e.g. the corrosion and wear resistance, the mechanical workability, etc.
Source: BAT Surface Treatment of Metals and Plastic, Aug. 2006.
